The present invention relates to a method of measuring the flow velocity of media in conduits by means of ultrasound, the ultrasonic waves being produced at one end of the measuring section by electric signals, transmitted through the measuring section and reconverted into electric signals at the other end. The invention further relates to a device for carrying out the method.
The well known idea, on which ultrasonic flow meters are based, is to send ultrasonic pulses through the flowing fluid, first in the flow direction and then in the opposite direction. Thereupon, the transit times of the two pulses are compared with each other.
As is known, the velocity of the ultrasonic pulses in the flow direction is higher and in the opposite direction lower than the sound velocity. The also known quantitative relation between the flow velocity v of the medium and the sound velocity c is given by the equation EQU c = c.sub.o .+-. v,
where c.sub.o is the sound velocity in the medium at rest. By measuring the difference of transit times in the two directions of sound propagation, it is possible to determine the flow velocity v of the medium. To effect this, either a time-difference or a frequency-difference method may be used.
In the time-difference method, the relation ##EQU1## IS APPLICABLE FOR THE SOUND TRANSIT TIME IN THE FLOW DIRECTION ALONG A SOUND PATH WITH THE LENGTH S AND THE RELATION ##EQU2## IS APPLICABLE FOR THE TRANSIT TIME AGAINST THE FLOW. Assuming that, in many practical cases, the flow velocity will be substantially lower than the sound velocity c.sub.o, the approximation equation is ##EQU3## The advantage of the time-difference method is a rapid response and the possibility, with a reversing switch, of using only one emitter and one receiver. It is disadvantageous, however, that small time differences must be measured and, in addition, that the result depends on the sound velocity in the medium. Thus, for determining the result, the sound velocity c.sub.o at the instant of measurement must be known.
For the frequency-difference method, two emitter-receiver couples are necessary in normal cases. In each couple, the pulse received at one end of the two transmission paths is used for releasing the next emitted pulse so that a pulse train is produced in each of the transmission paths. The period of this pulse train comprises the transit time. This circulation principle is known under the designation of "sing-around-method." The frequencies of recurrence are, for a pulse train in the downstream direction, ##EQU4## and, for a pulse train in the upstream direction, ##EQU5## wherefrom, the frequency difference is ##EQU6##
Thus, the frequency difference is proportional to the flow velocity of the medium, and independent of the sound velocity c.sub.o. The advantage of this method is an easily measurable quantity which is independent of the sound velocity. However, because of the long counting interval due to the multiple cycle, the response time is relatively long. In addition, two emitter-receiver couples are needed.
There is a known ultrasonic flowmeter in which sound pulses are transmitted either between two emitter-receiver units in both directions or between single emitters and receivers, respectively, also in both directions. As in the frequency-difference method, pulses are released in the circulating manner. The cycle terminates after a predetermined number of transmissions in either direction has been completed. The time difference between the respective received pulses in both transmission paths is measured. The time difference is increased by a factor corresponding to the number of recurrences. Such an increased time difference can be more easily measured with the desired accuracy. For this purpose, the time differences in the pulse trains are summed up. The disadvantage of this device is its dependence on the sound velocity in the medium at rest. Therefore, a possibility of correction must be provided (German Patent No. 2,110,582).
Another known flow meter determines the flow velocity of a fluid from the transit time of sound waves travelling through the fluid along three paths, namely along
a path extending obliquely upward, against the flow direction, PA1 a path extending obliquely downward in the flow direction, and PA1 a path extending obliquely downward in the flow direction, and a diametral path extending along a diameter of the pipe. Only one oblique path may also be provided. The transducers provided at the ends of this path are simultaneously excited by an electric signal having a duration shorter than the transit time of the sound waves, so that the two transducers can be operated both as emitters and receivers. The transit time difference of the signals is proportional to the flow velocity and to the cosine of the angle formed between the sound path and the flow direction.
Since this is a time-difference method, the measurement depends on the sound velocity, and the sound velocity at the instant of measuring is to be taken into account. This is done by a clock operating on the basis of a circuit loop in which the third sound path extending perpendicularly to the flow direction is connected. The operation of the clock is controlled as a function of the square of the sound velocity in the fluid (German Pat. No. 2,107,586).
A further known ultrasonic flow meter is based on the Doppler-frequency shift of a movable reflector. Sound waves are sent in the direction of foreign particles, such as dust particles, suspended particles, or the like, contained in the fluid to be measured. The foreign particles reflecting the ultrasonic waves have an acoustic impedance different from that of the fluid. The reflected sound waves are picked up by a receiver and the determined Doppler shift is used for determining the flow velocity of the fluid. To narrow the control range of the sound waves, devices, such as lenses, are provided in advance of the ultrasonic emitter or receiver, having to focus the ultrasonic waves.
The measuring is effected by means of an ultrasonic signal sent out by the emitter and having a frequency f.sub.o. The sound waves are reflected from the foreign particles in the fluid stream and picked up by the emitter at a frequency f.sub.r. The Doppler frequency EQU f.sub.d = f.sub.o - f.sub.r
is proportional to the velocity of the foreign particles and, consequently, to the velocity of the carrier fluid. The output signal representing f.sub.d can be indicated in an analog form or also in a digital form.
The disadvantage of this measuring method is the expensive equipment needed for the beaming of the sound waves as well as for the elimination of the temperature influence, which measures are necessary for accurate measurement, and the presence in the fluid of the particles necessary for the reflection of the sound waves (German Pat. No. 2,130,999).
Another known ultrasonic device for acoustic and flow measurements in fluids uses special modulation means for measuring the transit time of sound waves along at least one path extending through the fluid. The signal generator emits an oscillation which is frequency-modulated in accordance with an exactly predetermined principle and has a constant duration T. The difference of the transit times during the travel upwardly and downwardly is measured. The output signal representing this difference is measured by means of a timing frequency which is corrected as a function of the sound velocity in the fluid. The main drawback of this measuring device is the great bandwidth of the acoustoelectric transducers necessary for the method (German Pat. No. 2,109,222).